The effectiveness of adsorbents generally increases as their surface area and porosity increase. Even macroporous copolymers, which are prepared with a relatively high initial porosity and surface area, show improved adsorption, especially of small molecules, when their surface area is increased.
Reed, U.S. Pat. No. 4,263,407, and Reed et al., U.S. Pat. No. 4,191,813 disclose processes for increasing the surface area and porosity of both gel and macroporous, lightly crosslinked polymeric adsorbents. These processes form macronet structures by swelling the lightly crosslinked polymer with an organic solvent that swells but does not dissolve the copolymer, and post-crosslinking the swollen polymer using a Lewis-acid catalyst. The post-crosslinking step "freezes" the swollen structure in place, so that when the solvent is removed, the polymer is supported by the additional crosslinks. These prevent the polymer from shrinking to its original, unswollen volume, and allows it to maintain a total volume approaching that of the swollen polymer. As the solvent is removed, voids remain within the swollen structure which produce the increase in porosity and surface area. This process, termed "macronetting", depends upon swelling the polymer in a solvent, and consequently it depends heavily upon the degree of crosslinking present in the starting polymer. Highly crosslinked polymers swell poorly in solvents, so the increase in surface area achievable with such polymers is severely limited, according to the Reed disclosures.
Itagaki et al., in U.S. Pat. No. 4,543,365, discloses the use of a Lewis-acid catalyst to increase the porosity of swollen copolymers having a higher degree of crosslinking, including those with up to 80% crosslinker. As with Reed, Itagaki teaches and exemplifies the use of an organic swelling agent for the copolymer.
A desirable process for enhancing the porosity of copolymers would avoid the use of an organic solvent to swell the copolymers, as such solvents add elements of toxicity and waste disposal to the process.